I've had many oncidiums over the years now (much fewer) I would grow them in peatmoss with large corse rok. I would tend to focus in the middle area and add alot of perlite there to prevent any sorta rot issues. They grow fast with lots of spikes, they loved the moisture and I loved not having to water it only every few days.
They're very tough plants and can take drying out, but IMO they tend to prefer higher moisture levels and grow faster this way.
Sphagnum moss works well too. Just make sure they get good drainage. Airation & good drainage is the key to keep the roots healthy long term